The present invention is generally directed to an infrared detector, as well as to a process for the preparation thereof, which is suitable for use without the need of a cooling device. More specifically, the present invention is directed to an uncooled, infrared detector having a thermally isolated doped, organic carbon-containing sensor, formed by means of an implantation process.
Infrared (xe2x80x9cIRxe2x80x9d) or thermal imaging systems, or imaging systems employing infrared detectors or sensors, are important alternatives to visible light systems in applications where visible signals are either not available or not appropriate. Infrared systems can employ a single detector, such as in scientific instruments (e.g., IR spectrometers), or multiple detectors can be used in combination to form an array (e.g., a focal plane array or xe2x80x9cFPAxe2x80x9d) for use in military applications (e.g., imaging systems for individual soldiers or vehicles, weapons targeting systems or battlefield surveys) or civilian applications (e.g., driving aids, such as xe2x80x9cheads-upxe2x80x9d displays in vehicles, thermal imaging for energy efficiency audits, active sensors in security and safety systems and fire detection).
In general, infrared sensors or detectors operate by converting IR photons and energy into electrical signals. Infrared sensors or detectors can be divided into two broad categories. The first category, photon detectors, involve the direct interaction between the incident IR photons and electrons. These detectors are highly efficient because of the direction connection between the incident IR photons and the measured electrical response. Common examples of such detectors include photoconductive, photovoltaic, MIS and Schottky barrier devices.
The second category IR detectors, thermal detectors, involve the mediation of the interaction between the photons and electrons by phonons (i.e., thermal energy). More specifically, thermal detectors detect incident energy by means of an IR photon-temperature change-property change process. The detectors are made electrically sensitive to a property that is a function of temperature. A finite temperature change in the detector occurs as it absorbs IR radiation. This temperature change induces a property change in the material that can be measured by a corresponding electrical signal.
Thermal detectors are differentiated by the type of electrical signal that results from the temperature change. For example, pyro-electric sensors develop excess charge when exposed to temperature changes. Thermocouple sensors acquire a finite voltage, as a function of the temperature, across the sensor element. Bolometric sensors change electrical resistance as a function of temperature. Finally, capacitive sensors change dielectric constant as a function of temperature.
A number of features are important to the performance of an IR detector, such as a bolometer. For example, preferably the IR detector has a large responsivity. For bolometric sensors, it is recognized that typically responsivity is maximized when the detector has a large temperature coefficient of resistance (xe2x80x9cTCRxe2x80x9d), a small heat capacity (which enables the incident IR energy to result in a maximum temperature change), and a large thermal conductivity internal to the sensor. In the case of a FPA, preferably each individual sensor, or pixel, has a weak thermal link from the background or substrate to which it is attached, so that it can recover from a thermal event in a finite time. Additionally, to ensure the pixel reaches equilibrium before it is sampled by the electronics of the imaging system, a time constant limitation should also be met. More specifically, the thermal time constant, defined as the ratio of the heat capacity to the internal thermal conductance, should be kept below some fraction of the xe2x80x9cdwell time,xe2x80x9d which is generally defined as the time required to measure the electrical response of each pixel of the FPA.
Noise characteristics, or more specifically low noise characteristics, are another important feature for IR detectors. With respect to the noise limitation for focal plane arrays, it is recognized that this is related to the resistivity of each pixel or sensor. (See, e.g., Uncooled Infrared Sensor Performance, Infrared Technology, Proc. of SPIE, vol. 2020, 1993.) In some applications, for example, a resistance of 50,000 ohms is considered optimum for purposes of achieving the desired noise characteristics.
A high degree of uniformity is also important. In general, pixel or sensor spacial response uniformity has to do with how each pixel in the array compares to the others in terms of performance, and more specifically in terms of resistivity. Pixel uniformity is an important factor, for example, in achieving optimum performance in the overall array.
In addition to the above-noted performance features, it is also desirable for an IR detector to be easily fabricated, for example by means of standard semiconductor manufacturing techniques. Furthermore, it is also desirable, in many applications, for the detector to be capable of operation without the need of a temperature control or temperature regulating device of some kind; that is, it is desirable for the detector to be xe2x80x9cuncooled,xe2x80x9d or capable of operating at room temperature, because a cooling device adds weight and cost to the overall instrument.
Many approaches have been proposed to-date in an attempt to address each of the above-noted features. For example, in an attempt to develop IR detectors having improved responsivity and thermal isolation, many have proposed the use of bridge or cell structures, typically fabricated from polysilicon, upon which or from which a detector element is formed, either through doping of the bridge material or through deposition of a separate material such as a permalloy (e.g., NiFe) on the bridge. (see, e.g., U.S. Pat. Nos. 5,300,915; 5,260,225; 5,288,649; and 5,286,976.) However, such approaches fall short for a number of reasons, including difficulties in processing or preparation, poor uniformity or poor responsivity (e.g., poor TCR values). For example, it is extremely difficult to produce uniform films of materials commonly used for focal plane arrays, particularly uncooled FPAS, such as vanadium oxides (VOx) and barium strontium titanate (xe2x80x9cBSTxe2x80x9d), which typically have property variations of 10% or more.
Additionally, most technologies heretofore developed involve hybrid structures which present many difficulties in terms of being easy to manufacture, particularly in terms of reliability or reproducibility. Furthermore, while monolithic detectors or focal plane arrays have also been disclosed which have good uniformity (see, e.g., U.S. Pat. Nos. 5,900,799 and 5,629,665), in some instances high responsivity is achieved only by means of employing a temperature control device (see, e.g., U.S. Pat. No. 5,900,799).
Accordingly, a need continues to exist for an IR sensor material having, compared to existing materials, improved performance characteristics, as well as a process for the preparation thereof. Ideally, the sensor would be comprised of a material which is capable of forming a free-standing bridge-like structure having enhanced flexibility and strength, and the process would allow existing microlithographic or photolithographic techniques to be used to prepare the sensor, and preferably an array of such sensors.
Among the several objects of the present invention, therefore, is the provision of infrared detector, and a process for the preparation thereof, which employs a sensor having improved responsivity, including an improved temperature coefficient of resistivity, heat capacity and thermal conductivity; the provision of such a sensor having improved flexibility and strength, and which is capable of forming a free-standing, bridge-like structure; the provision of such a sensor which can be easily fabricated into a focal plane array using standard microlithographic techniques; the provision of such an array which has increased uniformity; the provision of such an array which has a high fill factor; the provision of such an array which can be used without the need of a temperature control device; and, the provision of such an array which enables the preparation of an improved imaging system, the system having decreased thermal noise and optimum resistivity.
Briefly, therefore, the present invention is directed to an infrared detector. The detector comprises (i) a substrate; (ii) a sensor having a segment thermally isolated from the substrate and a resistivity which varies as a function of temperature, the segment comprising an amorphous layer containing organic carbon and a dopant, the concentration of the dopant in the amorphous layer being at least about 1xc3x971020 atoms/cm3, the dopant being aluminum, an element having an atomic number greater than 18, or a mixture thereof; and, (iii) contacts electrically connected to the segment, the segment being energizable via said contacts.
The present invention is further directed to an infrared detector. The detector comprises (i) a substrate; (ii) a sensor having a segment thermally isolated from the substrate and a resistivity which varies as a function of temperature, the segment comprising an amorphous layer containing organic carbon and a dopant, the sensor having a responsivity of at least about 5000 V/W; and, (iii) contacts electrically connected to the segment, the segment being energizable via said contacts.
The present invention is still further directed to an infrared detector. The detector comprises (i) a substrate; (ii) a sensor having a segment thermally isolated from the substrate and a resistivity which varies as a function of temperature, the segment comprising an amorphous layer containing organic carbon and a dopant, the sensor having a temperature coefficient of resistivity of at least about 2%/K; and, (iii) contacts electrically connected to the segment, the segment being energizable via said contacts.
The present invention is still further directed to an infrared detector. The detector comprises (i) a substrate; (iii) a sensor having a segment thermally isolated from the substrate and a resistivity which varies as a function of temperature, the segment comprising an ion beam mixed layer containing organic carbon and a dopant other than carbon, the concentration of the dopant in the ion beam mixed layer being at least about 1xc3x971020 atoms/cm3, the ion beam mixed layer being formed by ion beam mixing of a target layer and an organic layer wherein the target layer comprises a source of the dopant; and, (iii) contacts electrically connected to the segment, the segment being energizable via said contacts.
The present invention is still further directed to processes for detecting infrared radiation using such detectors. In particular, the present invention is directed to a process for detecting infrared radiation comprising (i) passing an electrical current through a sensor of an infrared detector, the sensor having a segment thermally isolated from a substrate of the detector and a resistivity which varies as a function of temperature, the segment comprising an amorphous layer containing organic carbon and a dopant, the concentration of the dopant in the amorphous layer being at least about 1xc3x971020 atoms/cm3, the dopant being aluminum, an element having an atomic number greater than 18, or a mixture thereof; and, (ii) detecting a resistivity change in the sensor caused by the absorption of heat from infrared radiation impinging the sensor.
The present invention is still further directed to processes for preparing such infrared detectors. In particular, the present invention is directed to a process comprising (i) forming an organic layer on a substrate; (ii) forming a target layer comprising a dopant on the organic layer; and, (iii) contacting the target layer with an ion beam to form an infrared sensor on the substrate, the sensor having a segment thermally isolated from the substrate and a resistivity which varies as a function of temperature, the segment comprising an ion beam mixed layer containing organic carbon derived from the organic layer and a dopant other than carbon from the target layer, the concentration of the dopant in the ion beam mixed layer being at least about 1020 atoms/cm3.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.